source-file-types

Arguments:
ide-configuration

The value of this configuration option may be modified to change the
set of file types that the IDE recognizes as source code files. The
file types are used in places such as the file selection dialog, where
files of those types are presented when asking for a source code file.

The value should be a list of strings, where each string names a file
type. The default value is the list ("cl" "lsp" "lisp" "jil").

This option is not on the Options dialog, but could modified with
a form like the following. This example would add "mylisp" to the
list if it's not already there.